feat: add the okf mcp read tool surface

[chrishiguto]

Why

Agents need a sanctioned read surface that reproduces CLI and CI judgments without stale bundle state or protocol-channel noise.

What

okf mcp <bundle> serves four fixed tools over stdio: list-concepts, lookup-concept, query-graph, and validate. Every call re-reads the bundle. Stdout carries JSON-RPC only; diagnostics go to stderr.

validate returns the closed Spec report plus the adapter Verdict and supports the CLI-equivalent strict input. query-graph advertises OkfGraphQuery.jsonSchema verbatim and returns the versioned graph payload. All tools explicitly advertise read-only, non-destructive, idempotent, closed-world behavior.

Reviewer's guide

Start with lib/src/mcp/read_server.dart, then the real subprocess protocol harness in test/mcp_server_test.dart and the CLI wiring in lib/src/cli.dart.

Testing

• Formatting and fatal-info analysis are clean.
• All 156 tests pass.
• The stdio harness exercises every tool, Report/Verdict parity, exact graph-query schema reuse, bad inputs, partial bundles, pure JSON-RPC output, session recovery, and graceful EOF shutdown.
• MCP-only tests pass on Dart 3.4.0.
• dart pub publish --dry-run reports 0 warnings.

Links

Cleanly restacked on the merged prepared-change foundation.

Fixes #11.

[chrishiguto]

Rebased onto feature/16-bundle-change-set and retargeted, so the stack stays linear for the write verbs (#12/#13), which need this read surface and the change-set in one branch. No content change from the reviewed version — the rebase was clean and the suite is green at 111 tests on the new base.